To be honest if Hektor becomes a yandere it might be 7/10.
He may look like just a lazy and laid back middle-aged man, but remember he is actually a serious guy. He would prioritize his duty as Servant first, then his own ego and desire.
If you didn't have any romantic feelings for him, Hektor is okay with that because he respects your will. His love will turn into platonic affection. However, what would happen if you loved him back? Congratulations, you just made him the happiest man in the existence. Hektor swears in the name of his lance, that he would love and cherish you with his life.
Hektor doesn't mind seeing you interacting with another Servant and staff since you're the sole Master of Chaldea, but if they were too close with you he may give them some subtle warnings. But if that person is Achilles... Let's just say he may throw one or two rocks at him secretly.
On public Hektor may not show his affection towards you openly, he prefers to keep it private, but it doesn't mean he never show off. There was one time you got flirted by a random mob when you investigate a small Singularity, Hektor immediately pulled you closer and kiss you right on the lips in front of the mob. After he was done, he told the mob:
"Sorry, this person is mine already. Back off, would you?"
Even though he said it with light tone, there was danger laced on it. That mob got the message and immediately ran for life, didn't want to get his bad side.
What would happen if it were just you and Hektor alone? All of his attention would be yours, and there would be no interruptions unless it were an emergency call. He would crave your affection. He would spoil you with physical affection; kisses, hugs, anything you could ask for, he would provide, as long as you okay with it.
Even though Hektor respects your will, it doesn't mean he wouldn't resort to something extreme for your own good. Hektor desires nothing but your happiness and safety, after everything is over, he wants you to return to your old life, even if he had to say goodbye to you in the end. Unfortunately, the chance to return to your old life became slimmer or even zero, because of the sin you committed against the Lostbelts. To him, you don't deserve such an end. You were just like him when he was alive; an ordinary person who lived their life in peace and was dragged into a mess they didn't ask for. Hektor would do anything to protect you from a tragic end, even if that meant using the Holy Grail.
If he had to use the Holy Grail, what he wished for? He would wish to have you wishked away into a scheduled world, where it would be only for you and him. If the world refuses to allow you to live a peaceful and happy life, then he would create one himself.
"Even if it's just two of us, everything will be alright. Just be happy, alright, my sweetheart?"
